Suicides are far more likely to occur between midnight and 4 am than during the daytime or evening, a new study has found for the first time.

“This appears to be the first data to suggest that circadian factors may contribute to suicidality and help explain why insomnia is also a risk factor for suicidal ideation and behaviour,” said principal investigator Michael Perlis, from the University of Pennsylvania.

“These results suggest that not only are nightmares and insomnia significant risk factors for suicidal ideation and behaviour, but just being awake at night may in and of itself be a risk factor for suicide,” he said.

The researchers analysed a total of 35,332 suicides.

Results show that the weighted, scaled mean suicide rate per hour was 10.27 per cent after midnight, peaking at 16.27 per cent between 2 am and 2:59 am.

In contrast, the mean suicide rate per hour was 2.13 per cent between 6 am and 11:59 pm. When six-hour time blocks were examined, the observed frequency of suicide between midnight and 5:59 am was 3.6 times higher than expected.

According to the research authors, previous research suggesting that more suicides occur during the day failed to account for the proportion of the population that is awake at each given hour.

Perlis notes that an important implication of the study is that the treatment of insomnia may be one way to reduce suicide risk.